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The block diagram on the top shows the blocks common to all communication systems Communication systems Digital Analog

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Remember the components of a communications system: Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating Transmitter: The device that sends the transduced signal Transmission channel: The physical medium on which the signal is carried Receiver: The device that recovers the transmitted signal from the channel Output transducer: The device that converts the received signal back into a useful quantity

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Analog Modulation The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer – For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of Hz – The hello wave is a baseband signal:

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AM radioFM radio/TV Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation Remember the radio spectrum: For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band) The FM radio system must operate with frequencies in the range of MHz (VHF band)

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Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorily This process is accomplished by a device called a modulator The transmitter block in any communications system contains the modulator device The receiver block in any communications system contains the demodulator device The modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrum – For example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum – For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum) The demodulator extracts the original baseband signal from the received modulated signal To Summarize: Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal Modulation is done to bring information signals up to the Radio Frequency (or higher) signal

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Types of Analog Modulation Amplitude Modulation (AM) Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant Frequency Modulation (FM) Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant Phase Modulation (PM) Another form of analog modulation technique which we will not discuss

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Amplitude Modulation

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Uses a higher frequency carrier Most efficient use of frequency Time and Frequency Domain Susceptible to Noise

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S(t) = A c (1 + K a m(t))cos(2f c t) Where K a is a constant. m(t) is the baseband message. For an audio signal, the spectrum of m(t) would typically be in the range of 300 Hz - 3 kHz. f c is the carrier frequency m(t) = A m cos(2f m t) Where f c >> f m.

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The AM Waveform The three components that form the AM waveform are listed below: 1. The lower-side frequency (fc - fi) 2. The carrier frequency (fc) 3. The upper-side frequency (fc + fi) If a radio signal of frequency with1000KHz is mixed with an audio signal of 1 KHz the output will be A signal at 1000 KHz (Carrier wave) A signal at 1001 KHz (upper sideband) A signal at 999 KHz (lower sideband)

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Upper and Lower Sidebands

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In most systems the intelligence signal is a complex waveform containing components from roughly 200Hz to 3KHz. If this complex waveform is used to modulate the carrier there would be a whole band of side frequencies. The band of frequencies above the carrier is term the upper sideband. The band of frequencies below the carrier is called the lower sideband.

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Example A 1.4MHz carrier is modulated by a music signal that has frequency components from 20Hz to 10kHz. Determine the range of frequencies generated for the upper and lower sidebands.

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Points to be Remembered Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant The function of the carrier in AM is simply to provide a signal to heterodyne (mix) with the modulated audio, to convert all the AF components to a higher frequency. The bandwidth of an AM signal is equal to twice the highest freq. [ In commercial AM broadcast txs the freq of the modulating audio is permitted to be as high as 4.5 KHz, and since a double side system is used, commercial stations therefore have a bandwidth of 9 KHz] The bandwidth does not depend on the power of the modulating signal.